Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the “Air Pollution Complex”

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the“air pollution complex”was first proposed by Professor Xiaoyan TANG in 1997.For papers published in 2021 on air pollution(only papers included in the Web of Science Core Collection database were considered),more than 24000 papers were authored or co-authored by scientists working in China.In this paper,we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years,including studies on(1)sources and emission inventories,(2)atmospheric chemical processes,(3)interactions of air pollution with meteorology,weather and climate,(4)interactions between the biosphere and atmosphere,and(5)data assimilation.The intention was not to provide a complete review of all progress made in the last few years,but rather to serve as a starting point for learning more about atmospheric chemistry research in China.The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established,provided robust scientific support to highly successful air pollution control policies in China,and created great opportunities in education,training,and career development for many graduate students and young scientists.This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances,whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China,to hopefully be addressed over the next few decades.

The Significant Contribution of Small-Sized and Spherical Aerosol Particles to the Decreasing Trend in Total Aerosol Optical Depth over Land from 2003 to 2018

The optical and microphysical properties of aerosols remain one of the greatest uncertainties associated with evaluating the climate forcing attributed to aerosols.Although the trends in aerosol optical depth(AOD)at global and regional scales have been widely examined,little attention has been paid to the trends in type-dependent AODs related to aerosol particle properties.Here,using the aerosol optical component dataset from the Multi-angle Imaging SpectroRadiometer(MISR)instrument,we investigate decadal-scale trends in total aerosol loading as well as AODs for five aerosol components by particle size and morphology during 2003–2018 over land.Relationships between the total AOD(TAOD)trends and type-dependent AOD changes were examined,and the relative contribution of each type-dependent AOD to the overall TAOD trends was quantified.By dividing the TAOD values into four different aerosol pollution levels(APLs)with splits at 0.15,0.40,and 0.80,we further explored the relationships between TAOD changes and interannual variations in the frequency-of-occurrences(FoOs)of these APLs.Long-term trends in FoOs in the different APLs show that there was a significant improvement in air quality between 2003 and 2018 in most land areas,except South Asia,corresponding to a shift from lightly polluted to clean conditions.However,the effects of different APLs on TAOD changes are regionally dependent and their extent of correlation varied spatially.Moreover,we observed that the annual mean TAOD has decreased by 0.47%.a^(-1)over land since 2003(P<0.05).This significant reduction was mainly attributed to the continued reduction in small-sized(<0.7 mm diameter)AOD(SAOD)(-0.74%.a^(-1))and spherical AOD(SPAOD)(-0.46%.a^(-1)).Statistical analysis shows that SAOD and SPAOD respectively accounted for 57.5%and 89.6%of the TAOD,but contributed 82.6%and 90.4%of the trend in TAOD.Our study suggests that small-sized and spherical aerosols composed of sulfate,organic matter,and black carbon play a dominant role in determining interannual variability in land TAOD.

Aerosol Optical Properties and Radiative Impacts in the Pearl River Delta Region of China during the Dry Season

Aerosol optical properties and direct radiative effects on surface irradiance were examined using seven years （2006-2012） of Cimel sunphotometer data collected at Panyu--the main atmospheric composition monitoring station in the Pearl River Delta （PRD） region of China. During the dry season （October to February）, mean values of the aerosol optical depth （AOD） at 550 nm, the Angstrom exponent, and the single scattering albedo at 440 nm （SSA） were 0.54, 1.33 and 0.87, respectively. About 90% of aerosols were dominated by fine-mode strongly absorbing particles. The size distribution was bimodal, with fine-mode particles dominating. The fine mode showed a peak at a radius of 0.12 μm in February and October （- 0.10 μm3 μm-2）. The mean diurnal shortwave direct radiative forcing at the surface, inside the atmosphere （FATM）, and at the top of the atmosphere, was -33.4± 7.0, 26.1 ± 5.6 and -7.3 ±2.7 W m-2, respectively. The corresponding mean values of aerosol direct shortwave radiative forcing per AOD were -60.0 ±7.8, 47.3 ± 8.3 and -12.8 ±3.1 W m-2, respectively. Moreover, during the study period, FATM showed a significant decreasing trend （p 〈 0.01） and SSA increased from 0.87 in 2006 to 0.91 in 2012, suggesting a decreasing trend of absorbing particles being released into the atmosphere. Optical properties and radiative impacts of the absorbing particles can be used to improve the accuracy of inversion algorithms for satellite-based aerosol retrievals in the PRD region and to better constrain the climate effect of aerosols in climate models.

Applying the WRF Double-Moment Six-Class Microphysics Scheme in the GRAPES_Meso Model： A Case Study

This study incorporated the Weather Research and Forecasting（WRF） model double-moment 6-class（WDM6） microphysics scheme into the mesoscale version of the Global/Regional Assimilation and Pr Ediction System（GRAPES_Meso）. A rainfall event that occurred during 3–5 June 2015 around Beijing was simulated by using the WDM6, the WRF single-moment 6-class scheme（WSM6）, and the NCEP 5-class scheme, respectively. The results show that both the distribution and magnitude of the rainfall simulated with WDM6 were more consistent with the observation. Compared with WDM6, WSM6 simulated larger cloud liquid water content, which provided more water vapor for graupel growth, leading to increased precipitation in the cold-rain processes. For areas with the warmrain processes, the sensitivity experiments using WDM6 showed that an increase in cloud condensation nuclei（CCN）number concentration led to enhanced CCN activation ratio and larger cloud droplet number concentration（Nc） but decreased cloud droplet effective diameter. The formation of more small-size cloud droplets resulted in a decrease in raindrop number concentration（Nr）, inhibiting the warm-rain processes, thus gradually decreasing the amount of precipitation. For areas mainly with the cold-rain processes, the overall amount of precipitation increased; however, it gradually decreased when the CCN number concentration reached a certain magnitude. Hence, the effect of CCN number concentration on precipitation exhibits significant differences in different rainfall areas of the same precipitation event.

Modeling study on the roles of the deposition and transport of PM_(2.5) in air quality changes over central-eastern China

The role of PM_(2.5)(particles with aerodynamic diameters≤_(2.5)μm)deposition in air quality changes over China remains unclear.By using the three-year(2013,2015,and 2017)simulation results of the WRF/CUACE v1.0 model from a previous work(Zhang et al.,2021),a non-linear relationship between the deposition of PM_(2.5)and anthropogenic emissions over central-eastern China in cold seasons as well as in different life stages of haze events was unraveled.PM_(2.5)deposition is spatially distributed differently from PM_(2.5)concentrations and anthropogenic emissions over China.The North China Plain(NCP)is typically characterized by higher anthropogenic emissions compared to southern China,such as the middlelow reaches of Yangtze River(MLYR),which includes parts of the Yangtze River Delta and the Midwest.However,PM_(2.5)deposition in the NCP is significantly lower than that in the MLYR region,suggesting that in addition to meteorology and emissions,lower deposition is another important factor in the increase in haze levels.Regional transport of pollution in central-eastern China acts as a moderator of pollution levels in different regions,for example by bringing pollution from the NCP to the MLYR region in cold seasons.It was found that in typical haze events the deposition flux of PM_(2.5)during the removal stages is substantially higher than that in accumulation stages,with most of the PM_(2.5)being transported southward and deposited to the MLYR and Sichuan Basin region,corresponding to a latitude range of about 24°N-31°N.

Observation and analysis of near-surface atmospheric aerosol optical properties in urban Beijing

Year-round measurements of the mass concentration and optical properties of fine aerosols （PM2s） from June 2009 to May 2010 at an urban site in Beijing were analyzed. The annual mean values of the PM2.5 mass concentration, absorption coefficient （Ab）, scattering coefficient （Sc） and single scattering albedo （SSA） at 525 nm were 67 ± 66 μg/m^3, 64 ± 62 Mm^-1, 360±405 Mm^-1 and 0.82±0.09, respectively. The bulk mass absorption efficiency and scattering efficiency of the PM2.5 at 525 nm were 0.78 m^2/g and 5.55 m^2]g, respectively. The Ab and Sc showed a similar diurnal variation with a maximum at night and a minimum in the afternoon, whereas SSA displayed an opposite diurnal pattern. Significant increases in the Ab and Sc were observed in pollution episodes caused by the accumulation of pollutants from both local and regional sources under unfavorable weather conditions. Aerosol loadings in dust events increased by several times in the spring, which had limited effects on the Ab and Sc due to the low absorption and scattering efficiency of dust particles. The frequency of haze days was the highest in autumn because of the high aerosol absorption and scattering under unfavorable weather conditions. The daily PM2.5 concentration should be controlled to a level lower than 64 μg/m^3 to prevent the occurrence of haze days according to its exponentially decreased relationship with visibility.

Estimation and inter-comparison of dust aerosols based on MODIS, MISR and AERONET retrievals over Asian desert regions

This study presents detailed analysis of spatiotemporal variations and trend of dust optical properties i.e., Aerosol Optical Depth(AOD) and Angstrom component over Asian desert regions using thirteen years of data(i.e., 2001–2013) retrieved from Aerosol Robotic Network(AERONET), Moderate Resolution Imaging Spectroradiometer(MODIS) and Multi-angle Imaging Spectroradiometer(MISR). These regions include Solar Village, Dunhuang and Dalangzadgad and are considered as origin of desert aerosols in Asia. Mann–Kendall trend test was used to show the trend of AOD. The relationship of AOD with weather parameters and general AOD trend over different wavelengths has also been shown. AOD's trend has been observed significant throughout the year in Solar Village, while in Dunhuang and Dalanzadgad the significant trend has been found only in peak period(March–June).Analysis show high values of AOD and low values of angstrom in Solar Village during peak period. In Chinese desert regions, high values of AOD have been found during peak period and low values in pre-peak period. Significant relationship has been observed between AOD and average temperature in Solar Village and Dalanzadgad whereas rainfall and wind speed showed no significant impact on AOD in all desert regions.

Comparison of Two Air Pollution Episodes over Northeast China in Winter 2016/17 Using Ground-Based Lidar

This study analyzes and compares aerosol properties and meteorological conditions during two air pollution episodes in 19–22（E1） and 25–26（E2） December 2016 in Northeast China. The visibility, particulate matter（PM） mass concentration, and surface meteorological observations were examined, together with the planetary boundary layer（PBL） properties and vertical profiles of aerosol extinction coefficient and volume depolarization ratio that were measured by a ground-based lidar in Shenyang of Liaoning Province, China during December 2016–January 2017.Results suggest that the low PBL height led to poor pollution dilution in E1, while the high PBL accompanied by low visibility in E2 might have been due to cross-regional and vertical air transmission. The PM mass concentration decreased as the PBL height increased in E1 while these two variables were positively correlated in E2. The enhanced winds in E2 diffused the pollutants and contributed largely to the aerosol transport. Strong temperature inversion in E1 resulted in increased PM2.5 and PM10 concentrations, and the winds in E2 favoured the southwesterly transport of aerosols from the North China Plain into the region surrounding Shenyang. The large extinction coefficient was partially attributed to the local pollution under the low PBL with high ground-surface PM mass concentrations in E1,whereas the cross-regional transport of aerosols within a high PBL and the low PM mass concentration near the ground in E2 were associated with severe aerosol extinction at high altitudes. These results may facilitate better understanding of the vertical distribution of aerosol properties during winter pollution events in Northeast China.

RELATIONSHIP BETWEEN HORIZONTAL EXTINCTION COEFFICIENT AND PM_(10) CONCENTRATION IN XI'AN,CHINA,DURING 1980-2002

Based on daily visibility data obtained from 1980-2002 and air pollution index data from 2001-2004 in Xi＇an, long-term variations and relationships for daily horizontal extinction coefficient and mass concentration of PM10 have been evaluated. A decreasing trend was found in horizontal extinction coefficient during the past 23 years, with higher values observed in 1980s relative to 1990s, and the highest and lowest values in winter and summer, respectively. Significant correlation and similar seasonal variations existed between horizontal extinction coefficient and PM10 concentration, suggesting the high influence of PM10 to the visibility drop at a site in the Guanzhong Plain of central China during the past two decades.

Research Progress on Estimation of the Atmospheric Boundary Layer Height

Atmospheric boundary layer height(ABLH)is an important parameter used to depict characteristics of the planetary boundary layer(PBL)in the lower troposphere.The ABLH is strongly associated with the vertical distributions of heat,mass,and energy in the PBL,and it is a key quantity in numerical simulation of the PBL and plays an essential role in atmospheric environmental assessment.In this paper,various definitions and methods for deriving and estimating the ABLH are summarized,from the perspectives of turbulent motion,PBL dynamics and thermodynamics,and distributions of various substances in the PBL.Different methods for determining the ABLH by means of direct observation and remote sensing retrieval are reviewed,and comparisons of the advantages and disadvantages of these methods are presented.The paper also summarizes the ABLH parameterization schemes,discusses current problems in the estimation of ABLH,and finally points out the directions for possible future breakthroughs in the ABLHrelated research and application.

Optical and Radiative Properties of Aerosols during a Severe Haze Episode over the North China Plain in December 2016

The optical and radiative properties of aerosols during a severe haze episode from 15 to 22 December 2016 over Beijing, Shijiazhuang, and Jiaozuo in the North China Plain were analyzed based on the ground-based and satellite data, meteorological observations, and atmospheric environmental monitoring data. The aerosol optical depth at 500 nm was 〈 0.30 and increased to 〉 1.4 as the haze pollution developed. The Angstr6m exponent was 〉 0.80 for most of the study period. The daily single-scattering albedo was 〉 0.85 over all of the North China Plain on the most polluted days and was 〉 0.97 on some particular days. The volumes of fine and coarse mode particles during the haze event were approximately 0.05-0.21 and 0.01-0.43 μm^3, respectively-that is, larger than those in the time without haze. The daily absorption aerosol optical depth was about 0.01-0.11 in Beijing, 0.01-0.13 in Shijiazhuang, and 0.01-0.04 in Jiaozuo, and the average absorption Angstrom exponent varied between 0.6 and 2.0. The aerosol radiative forcing at the bottom of the atmosphere varied from -23 to -227, -34 to -199, and -29 to -191 W m^-2 for the whole haze period, while the aerosol radiative forcing at the top of the atmosphere varied from -4 to -98, -10 to -51, and -21 to -143 W m^-2 in Beijing, Shijiazhuang, and Jiaozuo, respectively. Satellite observations showed that smoke, polluted dust, and polluted continental components of aerosols may aggravate air pollution during haze episodes. The analysis of the potential source contribution function and concentration-weighted trajectory showed that the contribu- tion from local emissions and pollutants transport from upstream areas were 190-450 and 100-410 btg m-3, respectively.

Spatial and temporal distribution of the cloud optical depth over China based on MODIS satellite data during 2003–2016

The cloud optical depth(COD) is one of the important parameters used to characterize atmospheric clouds. We analyzed the seasonal variations in the COD over East Asia in 2011 using cloud mode data from the AERONET(Aerosol Robotic Network) ground-based observational network. The applicability of the MODIS(Moderate Resolution Imaging Spectroradiometer) COD product was verified and compared with the AERONET cloud mode dataset. There was a good correlation between the AERONET and the MODIS. The spatial and temporal distribution and trends in the COD over China were then analyzed using MODIS satellite data from 2003 to 2016. The seasonal changes in the AERONET data and the time sequence variation of the satellite data suggest that the seasonal variations in the COD are significant. The result shows that the COD first decreases and then increases with the season in northern China, and reaches the maximum in summer and minimum in winter.However, the spatial distribution change is just the opposite in southern China. The spatial variation trend shows the COD in China decreases first with time and gradually increases after 2014. And the trend of COD in the western and central China is consistent with that in China. While the trend of COD shows a continuously increasing over time in northeast China and the Pearl River Delta.

Development of an integrating sphere calibration method for Cimel sunphotometers in China aerosol remote sensing network

Based on the integrating sphere traced from the National Institute of Standards and Technology （NIST, USA）, a sphere calibration method and protocol for the China aerosol remote sensing network （CARSNET） Cimel sun photometer was established. Four CE318 sun photometers were verified using the proposed cal- ibration method and operational protocol. The calibration results showed that the instrument coefficients differed by less than 3% for visible （-5% for infrared） wavelengths from the original ones stated by Cimel Electronique. In situ validation experiment data showed that radiances at ±6° measured by sun collimator （aureole） were consistent with those measured by sky collimator （sky）, under both almucantar （ALMUC） and principal plane （PPLAN） scenarios. Differences at all wavelengths were less than 1%, indicating that the method and protocol are suitable for CARSNET field sun photometer calibration, and would benefit improvement of data quality and accuracy of network observations.

2022汤加火山灰羽流的气溶胶光学、微物理和辐射特性

作为21世纪乃至近30年最强的火山喷发之一,2022年汤加火山喷发事件已经引起了广泛关注.研究表明,汤加火山喷发所产生的大量火山灰气溶胶突破对流层进入平流层,形成了一个顶部高度约为25~30 km的火山灰羽流.在喷发后的4天内,火山灰羽流在平流层环流的驱动下迅速向西移动了近10000 km.侵入平流层的火山灰气溶胶导致整个澳大利亚北部的大气气溶胶负荷显著增加,气溶胶光学厚度(AOD)在澳大利亚东北部海岸达到1.5,约为侵入前一日的15倍.此次汤加火山灰羽流主要以半径集中在~0.26μm处的细模态颗粒物为主,同时其体积峰值达到0.25μm^(3)μm^(-2).汤加火山喷发对平流层AOD和辐射平衡的影响显著,卫星观测到的平流层AOD的扰动高达0.6.这种扰动在很大程度上解释了区域性地表(大气层顶)的瞬时短波辐射强迫可达-105.0 W m^(-2)(-65.0 W m^(-2)).

RELATIONSHIPS BETWEEN DUST STORMS AND DRYNESS-WETNESS IN MIDDLE–EASTERN CHINA DURING 1470–1950

Based on 481-year records of historical dust storm （DS） and Dryness-Wetness Index （DWI） at 120 sites, spatial distribution characteristics of dryness-wetness （DW） in typical dust storm years （DS years） and in non-dust storm years （non-DS years） were derived for continental China. In DS years, most of the sites were drier than in normal years while in non-DS years wetter than normal, and the variation of DWI in DS years was larger than that in non-DS years. The relative instability and increased regional difference of atmospheric circulation in DS years might have induced more frequent DS events and dry-wet abnormality in continental China. In DS years the latitudinal （north-south） dry-wet difference was larger than that in non-DS years, that is, north China was even much drier than south China. This might be attributed to increased latitudinal differences of thermal and pressure gradients in DS years, resulting in the southward withdrawal of precipitation and increase of DS events.

A new method to retrieve aerosol optical thickness from satellite images on a parallel system

A wide variety of algorithms have been developed to monitor aerosol burden from satellite images. Still, few solutions currently allow for real-time and efficient retrieval of aerosol optical thickness （AOT）, mainly due to the extremely large volume of computation necessary for the numeric solution of atmospheric radiative transfer equations. Taking into account the efforts to exploit the SYNergy of Terra and Aqua Modis （SYNTAM, an AOT retrieval algorithm）, we present in this paper a novel method to retrieve AOT from Moderate Resolution Imaging Spectroradiometer （MODIS） satellite images, in which the strategy of block partition and collective communication was taken, thereby maximizing load balance and reducing the overhead time during inter-processor communication. Experiments were carried out to retrieve AOT at 0.44, 0.55, and 0.67μm of MODIS/Terra and MODIS/Aqua data, using the parallel SYNTAM algorithm in the IBM System Cluster 1600 deployed at China Meteorological Administration （CMA）. Results showed that parallel implementation can greatly reduce computation time, and thus ensure high parallel efficiency. AOT derived by parallel algorithm was validated against measurements from ground-based sun-photometers; in all cases, the relative error range was within 20%, which demonstrated that the parallel algorithm was suitable for applications such as air quality monitoring and climate modeling.